The present invention relates generally to grids and their formation, and more particularly to field extraction grids and their construction for field emission displays.
In the microelectronics industry, there is a movement toward creating flat panel displays. These displays have the advantage of being significantly more compact than cathode ray tube displays, e.g., conventional computer monitors. There are different types of flat panel displays, such as liquid crystal displays (xe2x80x9cLCDsxe2x80x9d), gas-plasma displays, thin film transistor (xe2x80x9cTFTxe2x80x9d) displays, and field emission displays (xe2x80x9cFEDsxe2x80x9d). FEDs are particularly well suited to applications requiring high resolution, low power demand, wide viewing angle, and physical robustness in an operational environment.
FEDs are able to achieve high resolution owing in part to the presence of a significant number of emitter tip structures concentrated in a small space. These emitter tip structures, or cold cathode field emitter tip structures, and their formation are described in U.S. Pat. Nos. 5,391,259, 5,372,973, 5,358,908, 5,151,061, 3,755,704, 3,665,241, among others.
For emitter tip structures to emit electrons, a voltage bias is applied across the emitter tip structures and an extraction grid to create a potential difference therebetween. In U.S. Pat. No. 5,372,973 to Doan et al., formation of an extraction grid self-aligned to emitter tip structures is described.
In Doan et al., after forming emitter tip structures, a silicon nitride layer is deposited over the emitter tip structures. This layer is conformal to the surface upon which it is deposited. Next, boro-phospho-silicate-glass (xe2x80x9cBPSGxe2x80x9d) is deposited as an insulating layer. The BPSG layer is deposited and re-flowed, such that it does not extend above the silicon nitride layer. In other words, the silicon nitride layer above the emitter tip structures is left exposed after deposition and re-flowing of the BPSG. Next, a conductive layer, such as a layer of polysilicon having impurities (xe2x80x9cdopantsxe2x80x9d), is deposited on the BPSG layer and the exposed regions of the silicon nitride layer. The layer of polysilicon is chemically-mechanically polished to re-expose regions of the silicon nitride layer; specifically, those regions disposed above apexes of the emitter tip structures. Accordingly, the polished conductive layer of polysilicon forms an extraction grid self-aligned to the emitter tips. The assembly may then be etched to pull the silicon nitride and the BPSG away from the emitter tip structures.
Though Doan et al. provide a self-aligned process for forming an extraction grid after formation of emitter tip structures, Doan et al. exposes the extraction grid layer to water, chemical-mechanical-polishing (CMP) slurry, and other potentially corrosive materials, some of which must then be cleaned off the assembly with other materials which may be harmful to some emitter structures.
A technique known as xe2x80x9cetch backxe2x80x9d is an alternative to CMP in situations where a blanket flow fill layer is previously deposited. Etch-back typically refers to a blanket plasma (xe2x80x9cdryxe2x80x9d) etch of such a surface. Etch-back does not have the above-mentioned disadvantages of CMP. However, etch-back uniformly removes material across a surface. Referring to U.S. Pat. No. 5,266,530 to Bagley, et al. (xe2x80x9cBagleyxe2x80x9d), dielectric layer 24 is etched back to expose a portion of underlying dielectric layer 22. Dielectric layer 22 may then be etched to pull it away from tip 18. Gate layer 26 may then be deposited, and subsequently etched to remove a portion of gate layer 26 deposited on tip 18. In Bagley, uniform removal by etching is employed. However, it would be desirable to define a gate layer with fewer etching steps than Bagley.
Accordingly, it would be desirable in the art of manufacturing field emission devices to provide a self-aligned process for forming an extraction grid after forming emitter tip structures with the advantages associated with dry etch with conformal or substantially conformal (with plus or minus 50 nm) deposit material using fewer etch steps than in Bagley.
The present invention provides a method for forming a grid. In particular, a substrate assembly having one or more emitter tip structures formed thereon or therefrom is provided. An insulative layer is formed on or above the emitter tip structures, as well as on or above an associated emitter layer from which the emitter tip structures protrude. A conductive layer is formed on or above the insulative layer. An exposed surface of the conductive layer thus exhibits topographical variation owing to the presence of the underlying emitter tip structures. The exposed surface is then subjected to particle bombardment from ion milling. These particles are used to remove material from the conductive layer at various etch rates dependent at least in part on angle of incidence thereof. More particularly, portions of the conductive layer in near proximity to the one or more emitter tip structures are removed more rapidly than other portions. Accordingly, the insulative layer may be exposed in near proximity to the one or more emitter tip structures, while leaving a surrounding portion of the conductive layer for forming the grid.
In accordance with the present invention, a grid structure may be formed. Such a grid may be used as an anode in a field emitter display device for extracting electrons from emitter tip structures, namely, as an xe2x80x9cextraction grid.xe2x80x9d Such an xe2x80x9cextraction gridxe2x80x9d may be formed self-aligned or centered to one or more of the emitter tip structures due to the preferential etching of the conductive layer overlying the emitter tip structure locations. In other words, the extraction grid or portions thereof may have z-axis (an axis traveling up through the center of an emitter tip structure) symmetry with respect to one or more associated emitter tip structures. Stated another way, a portion of the extraction grid in near proximity to an associated emitter tip structure is centered relative to said structure. Owing to performance characteristics dependent upon alignment of an emitter tip structure and its corresponding anode extraction grid section, as well as ease of manufacture, a self-aligned process for forming such an extraction grid is advantageous. Moreover, an extraction grid in accordance with the present invention may be formed in-situ with respect to other portions of the field emission display. Furthermore, ion milling may be used to expose the one or more emitter tip structures for sharpening. Such sharpening may be done in-situ with the ion milling used to expose the emitter tip structures.